This proposal is focused upon the intracellular amastigote stage of Leishmania parasites and in particular, member of the New World L mexicana complex. Disease pathology associated with infection is due to mechanisms of an response by the host to the amastigote stage of the parasite. Research during the past period of support has demonstrated that immunization with molecules specifically expressed or up-regulated in the amastigote stage can confer resistance to infection. Resistance/protection against infection in C57BL/6 mice appears to depend upon CD4+ as well as CD8 + T cell sub-populations. Interestingly, and in apparent conflict with protective immunity results, recent data indicate that amastigotes residing within the macrophage parasitophorous vacuole ( phagosome) can sequester their antigens from class II MHC presentation. Consequently, the work proposed in the current application will: 1. Characterize the protective immune response induced by immunization with the P-4 and P-8 Leishmania amastigote antigens. The contributions of CD4+ and CD8+ T cell to protection against infection will be examined in mice of different genetic backgrounds. Given the demonstrated role of CD8 T cells in the protective response of immune C57BL/6 mice the potential mechanisms of CD8 T cell modulation of the host immune response will be investigated. 2. Investigate the biological function of the P-4 amastigote protein and molecularly clone/characterize the P-8 amastigote molecule. 3. Determine the potentially protective cross-reactive eptiopes of the P-4 and P-8 amastigote proteins. Given the exclusive Th 1-like response [CD4 + and CD8 +] observed in L. braziliensis infected individuals to the . pifanoi derived P-4 and P-8 molecules, the cross-reactive eptiopes responsible will be determined. If "clustered" or a limited set of determinants is found, the efficacy of a peptide-based vaccine will be evaluated using a murine model. 4. Examine antigen presentation by Leishmania amastigote infected M phi's. This specific aim is primarily focused on MHC class I presentation. Using CD8+ T cell lines the time course, nature and level of class I presentation of both protective and non-protective amastigote antigens will be examined. The role of specific amastigote-M phi ligands in the mediation of MHC class II sequestration will also be examined. This proposal is focused on elucidating and characterizing mechanisms involved in parasite immune evasion as well as effector mechanisms involved in protection against infection, in an attempt to further understand the ongoing events regulating critical aspects of the host- Leishmania interaction.